This invention is directed to a method of producing steels having an ultra-low carbon and ultra-low sulfur content which are suitable for use as electrical steels, and more particularly as motor lamination steels (MLS).
For many uses, such as motor lamination, today's electrical steels must be of a high grade, with residual elements such as carbon, sulfur and nitrogen being significantly eliminated from the steel. A reduction in the carbon content helps to prevent or limit the effects of magnetic aging and lowers core loss. The reduction of sulfur in the steel helps to eliminate MnS inclusions, thereby improving core loss. Thus, the chemistries for MLS and other high grade electrical steels are ideally characterized by ultra-low carbon and sulfur contents of less than about 0.005% each.
Typically, ultra-low carbon processing involves refining in a basic oxygen furnace (BOF) followed by vacuum decarburization to ultra-low carbon levels. As is known in the art, basic-oxygen processes typically involve the charging of molten iron, steel scrap and other components for the formation of the liquid steel into a metallurgical vessel and blowing a high velocity stream of oxygen from a lance into the molten ferrous starting materials to refine them into steel. The details of basic oxygen processes in general, and of the Basic Oxygen Furnace (BOF) in particular, are well known to those of ordinary skill in the art.
Similarly, as is known in the art of manufacturing ultra-low carbon steels, the carbon content of the melt is typically reduced to ultra-low levels by a vacuum circulation process (VCP) in a so called vacuum degasser. In the vacuum decarburization process the melt is introduced into a low pressure environment so that carbon and oxygen are evolved out of the melt as gaseous reaction products such as carbon monoxide. Inert gas is introduced into the melt, typically through tuyeres submerged in the bath, to reduce the partial pressure of the CO and to agitate and stir the bath. An important concern in the manufacture of ultra-low carbon steels is the problem of carbon pick-up after satisfactory carbon levels have been obtained. Carbon-bearing materials such as alloying agents, slag deoxidants, ladle refractories and graphite electrodes used in the ladle furnace can contaminate the steel, making it difficult to maintain the required ultra-low carbon levels. Accordingly, it has been the practice to avoid further processing after decarburization to ultra-low levels, and to cast the steel into solid shapes as soon as possible after decarburization.
Desulfurization is typically carried out in a ladle prior to vacuum decarburization in order to avoid additional processing and associated carbon pick-up after degassing. Occasionally, desulfurization is carried out subsequent to deoxidation, but while the melt is still under vacuum in a degassing vessel. These processes typically involve the introduction of a desulfurizing reagent or flux into the molten metal charge to remove the sulfur from the steel. Since many of the components of the ladle slag such as FeO and MnO are deleterious to the desulfurization process, it is usually desirable to keep the ladle slag from intermixing with the molten metal being refined unless the slag is treated or replaced with an artificial slag that will not hinder the desulfurization process. Moreover, to conduct both decarburization and desulfurization while under vacuum, the apparatus must be constructed to enable the desulfurizing flux to be injected during the vacuum process.
The present invention provides a method of producing ultra-low carbon, ultra-low sulfur steel which advantageously avoids the need for costly desulfurizing reagents. Its ability to utilize a conventional ladle furnace for desulfurization avoids the need for the complex apparatus necessary to enable simultaneous vacuum circulation and desulfurization. The particular order of the inventive processing steps renders the inventive method particularly useful in continuous casting operations, and the effects of carbon pick-up after decarburization can be controlled.